1. Field of the Invention
The present invention relates to a bellows tube, more specifically the bellows tube for use in an exhaust system of an automotive vehicle.
2. Description of the Related Arts
In general, a flexible tubular joint is employed in a vehicle exhaust system at a connecting portion of exhaust pipe, so as to absorb a vibration of an engine or a relative displacement between the engine and the exhaust pipe. Once a bending moment or shearing stress is applied to the flexible tubular joint, which may be called as a flexible tube, the moment and stress applied thereto will be the maximum at an end portion of the flexible tube, i.e., the connecting portion. In order to improve durability of the tube, various countermeasures have been proposed, heretofore. In Japanese Patent Laid-open Publication No.8-28260, for example, disclosed is a flexible tube which is formed of a metal tube 21 having a bellows portion 23 with a mountain portion 25 and a valley portion 27 formed alternately, wherein a circular arc portion 29 is formed on the top of the mountain portion 25 and the bottom of the valley portion 27, respectively. According to the Publication, it is proposed that the heights of the fourth mountain M4 and following mountains are made equal, and the heights of the third mountain M3, second mountain M2 and first mountain M1 are gradually lowered toward the end of the bellows portion 23, and that the radius of curvature of the circular arc portion 29 formed at a rising portion 31 of the first mountain M1 is made more than two times as large as the radius of curvature of the circular arc portion 29 formed at each top of the fourth mountain M4 and following mountains.
In Japanese Utility Model Laid-open Publication No.1-166886, with respect to a tube connecting apparatus for connecting one tube with the other one tube through a bellows tube, and protecting the outer side of the bellows tube by an outer protecting cylinder, it is proposed that a large diameter portion of the outermost end the bellows tube is made longer than other large diameter portions.
In Japanese Patent Laid-open Publication No.61-235025, there is disclosed a method for manufacturing an .OMEGA.-type bellows, which includes a first process for forming a tube blank to provide a U-shaped mountain portion and a straight tube portion crossed normal to the bottom of the mountain portion, with a predetermined space formed between them, a second process for compressing the tube blank in its axial direction by a predetermined length, with an inside pressure applied to the tube blank formed by the first process, and a third process for applying high pressure into the tube blank for a predetermined period, with the tube blank compressed.
According to the flexible tubular joints disposed in the exhaust pipe of the vehicle including the flexible tube as disclosed in the publications listed above, outwardly curved portions of a metal tube (designated as the mountain portion in the Publication No.8-28260) which are curved outward of the metal tube, and inwardly curved portions (designated as the valley portion in that Publication) which are curved inward of the metal tube, are formed to provide a generally .OMEGA.-shaped cross section, respectively, to improve a resiliency.
In the Publication No.8-28260, it is aimed to lessen a concentration of stress to the end portion, by enlarging the radius of curvature of the circular arc portion formed at the rising portion of the first mountain. As a result, a damage can be avoided at the rising portion, but a possible damage may be made at a portion near the top of the first mountain. This is because, the mountain portion of the first mountain is formed on the metal tube by a general bulging process, so that the thickness of the bulged first mountain is reduced comparing with that of the metal tube before it is bulged, and also because an inflection point is made on a connecting portion between the first mountain and the end of the rising portion following the first mountain, to weaken a portion at the point. Furthermore, if a bending deformation is made on the flexible tube, the stress will be concentrated on the portion near the top of the first mountain. Therefore, the tube is likely to be broken at the inflection point.